November 2010 www.Agg-Net.com 13

Safe and SoundRock and soil slope stabilization and protection

By David Gibson, business development manager, BAM Ritchies

The safety of people, infrastructure andfacilities below slopes is important.Technology and working methods for

slope stabilization and protection systemshave advanced significantly over the lastcouple of decades, thus improving slopesafety.While the construction industry tends to

lead with the advances being made, as theprotection of the public and infrastructure isparticularly critical, the solutions nowavailable are equally applicable to theextractive industries.With regard to the stability and safety of

excavations and tips, the requirements of theQuarries Regulations 1999 are clear in respectof the need for ‘geotechnical assessment’ andthe operator’s duties in relation the thesegeotechnical assessments (Regulation 33).Often the stability of excavations and tips

can be ensured by scaling, profiling orregrading. However, this is not always possibleand the use of alternative techniques, such aspre-split blasting, rock bolts, ground anchors,soil nails, meshing or even rockfall barriers,is sometimes required to ensure the safety ofthe excavation or tip slope faces and thepersonnel, infrastructure and assets inoperation below them.Since these situations occur in both the

construction and the extractive industriesworldwide, the experience and techniquescurrently used are applicable to the UKquarrying industry.

AApppprraaiissaall aanndd aasssseessssmmeennttBefore any solution is developed andproposed, appraisal and assessment arevitally important to ensure that sitecircumstances and risks are properlyunderstood and lead to adequate economicsolutions with long-term serviceability. These,and the resultant remedial solutions, must becarried out by competent engineers. It is

notable that a UK register for groundengineering professionals was due to belaunched in September 2010.Good assessments should provide a clear

understanding of the hazard and risks involved

and lead to economic solutions. Poorassessments may well result in inadequatesolutions or, conversely, over-engineeredand expensive stabilization and protectionmeasures.The use of stereo photography, laser

scanning and other modern techniques canassist in good understanding of the slope inquestion and can be used to supplement oreven replace physical inspection and appraisal(including rope access), which may behazardous in itself.

PPrreeppaarriinngg ffoorr rreessiidduuaall ssllooppeessIt can be very worthwhile for quarry operatorsto plan in advance their residual faces beforethey reach them. A clear understanding of thegeology leading up to, at and behind theplanned residual face can assist with theproper design of that face and reduce oreliminate long-term instabilities, hazards orexpensive protection measures. �

Slope face inspection

14 www.Agg-Net.com November 2010

Carefully controlled blasting and the use ofpre-splitting, taking into account both thegeology and the final profile of the quarry, arelikely to be cost-effective in the long run.

SSttaabbiilliizzaattiioonn aanndd pprrootteeccttiioonnssoolluuttiioonnssWhere it is not possible to eliminate hazardsby blasting, scaling or re-profiling, there areother solutions available. These are constantlyevolving, as are the associated installationtechniques.For slope stabilization, these solutions

include:—passive netting (drape netting)—active support meshes—rock bolts—ground anchors—soil nails.

Protection techniques include:—rockfall attenuators—rockfall barriers—galleries—debris-flow barriers—landslide barriers.

SSllooppee ssttaabbiilliizzaattiioonn Rock bolts and ground anchors, soil nailingand slope face meshes have been in use formany years (indeed, it is thought that the greatinnovator Isambard Kingdom Brunel used soilnails near Bristol Temple Meads Station in themid-19th century). Recent improvements inthe understanding of these processes hasseen growth in geotechnical solutions toslope stability problems. Access andinstallation methods have also moved forwardsignificantly.The CIRIA ‘Soil Nailing – Best Practice

Guidance’ and EN 14490 ‘Execution of SpecialGeotechnical Works’ documents have assistedin the advancement of the subject. Moderndrilling methods and nail materials allowrobust, cost-effective, long-life slopestabilizations, while increased use of safe ropeaccess facilitates work on steep rock slopes.Modern high-performance top-hammer

drill rigs, such as the Atlas Copco D7, can be

used for both blast drilling and geotechnicalworks, and where the two activities are takingplace together the use of this type of multi-purpose unit means that one rig can do bothactivities, thereby saving the client time andmoney compared with having to mobilizetwo different types of rig.Where slopes are high or awkward to

reach the use of high-production hydraulicfeed beams mounted on high-reachexcavators, often using umbilical controls,allows safe installations.Drape nettings and shallow slope-

stabilization facings typically use readilyavailable hexagonal-twist wire meshes. Now,higher-angled slopes and those with less-cohesive ground conditions can be stabilizedby high-tension steel meshes, such asGeobrugg’s TECCO and SPIDER meshes,sometimes replacing more expensive sprayedconcrete and lacing systems respectively.

These high-tension steel meshes providea positive active support, unlike passive un-tensioned meshes which just contain loosematerial after it has become dislodged.Tensioned active support meshes do notrequire longer-term maintenance throughbleeding and reinstatement.TECCO mesh is used on both soil and

rock slopes, and owing to its high strength canreduce the number of soil nails or rock bolts,so reducing overall costs. SPIDER mesh isused to secure rock slopes that have largerunstable blocks. Both products have been fullytested by third party test houses and are nowin widespread use throughout the world.Software is also available to assist with the

design of these cutting-edge systems. Theseprograms are simple to use by a trainedgeotechnical designer and allow easy designadjustments on site to accommodatecircumstances that may not have been fully

Holme Park pre-split Dawlish cliff stabilization

Rock face stabilizations in Scotland using tensioned and un-tensioned meshes

November 2010 www.Agg-Net.com 15

apparent during the initial geotechnicalassessment.Should a final ‘green’ finish be required,

erosion-control mattings, along with brushed-in topsoil and hydroseeding, may be usedunder the meshes to provide long-term,aesthetically pleasing solutions, oftenreducing the visual impact of the quarryingoperations.If it is intended to use rock bolts, ground

anchors or soil nails close to the quarryboundary, care is needed to ensure that theydo not trespass into third party properties,otherwise permission or easements will berequired. While rock bolts and, to a certain degree,

ground anchors are sometimes used inquarries, especially when installed inconjunction with facing meshes, the use of soilnails is relatively new.Soil nailing is an earth-reinforcement

technique that is now widely used in theconstruction industry for soil slopestabilization, often instead of expensiveretaining walls. Applications in the quarryingindustry allow secure steep overburdenslopes, not only ensuring their safety butpossibly also permitting the release ofreserves that would otherwise be sterilized.In one recent example where there was anunstable overburden slope, soil nailing wasused to remediate it and reserves werereleased, which more than paid for the costof the stabilization.

RRoocckkffaallll pprrootteeccttiioonnOften, the use of rock bolts, ground anchorsand meshes can reduce or eliminate risks.However, it is not always practical or economicto use these owing to the face size, topographyor vegetation. If there are large areas to bestabilized the cost may be prohibitive. Analternative solution may be a rockfall barrier. On 1 February 2008 the official EU Guideline

for European Technical Approval of FallingRock Protection Kits – ETAG 027, produced byEOTA – European Organisation for TechnicalApprovals, became the key recognized design,test and manufacturing standard for EUcountries.

Soil nailing a quarry overburden slope

Rockfall barrier installed in Swinden Quarry, North Yorkshire

Barriers in full compliance with theseguidelines are now becoming

available. Barriers in the rangeof 500kJ to 5,000 kJ will soonhave official ETAs (EuropeanTechnical Approvals) and theconsequent CE marking. Thecurrent status may be seenat: www.eota.eu > Valid ETAs> criteria: ETA Number. Atthe time of writing onlyGeobrugg GBE barriers wereofficially recognized by EOTA.The development and

innovation of these newbarriers means that they arenotably simpler and lighterthan previous designs, so areeasier and quicker to install.BAM Ritchies have recentlyinstalled the first ETAG 027-

generation barrier at a site in Scotland. In order to decide on the suitability of a

rockfall barrier, it is important that a full andproper assessment is made to ensure that thebarrier is correctly specified and installed.Initially, the hazard needs to be understood,so that the risk may be evaluated. This isfollowed by the decision on the preference forface remediation or barrier installation. Todesign the barrier, the largest potential blocksize, its location and the topography of the faceall need to be determined. Only then cancomputer simulation be carried out todetermine the maximum likely impact energyand bounce height at the barrier location,thereby defining the barrier design.Installation involves staking out the barrierlocation and deciding on the foundations andanchorages.The latest evolution of barriers are

lightweight and easy to install, and, if �

Latest-generation barrier installation in Scotland

GGeeootteecchhnniiccss

16 www.Agg-Net.com November 2010

GGeeootteecchhnniiccss

long-term durability is important, high-qualitycorrosion protection can be provided.

DDeebbrriiss fflloowwssDuring the course of the development and useof the latest flexible rockfall barriers,sometimes it was noted that they not onlycaught falling rocks but also debris flows.Debris flows typically consist of a significantnumber of boulders and finer materialmobilized by high volumes of water.The loadings on debris flow barrier

systems comprise higher volumes but usuallyat lower velocities. As a result, the barriers,while appearing to be similar to rockfallbarriers, are of a different design with groundanchors able to take the higher overallloadings.The WSL organization in Switzerland,

assisted by barriermanufacturer Geobrugg,has undertaken years ofresearch and developmentto ensure that barriers arenow designed properly andable to provide realisticmitigation. Advanced flexibledebris flow barriers are nowbeing installed worldwide; in particular in alpinecountries, as well asCalifornia, Japan and Korea.In the UK, a debris flow

barrier has recently beeninstalled in a gulley close toa landslide barrier at theRest-and-be-Thankful pass,with the aim of preventing repeats of thecircumstances that have already closed theA83 in recent years by eliminating the chanceof the culvert under the road becomingchoked with debris.

LLaannddsslliiddee bbaarrrriieerrssWhile rockfalls tend to involve discrete blocksfalling at high velocities and debris flows tendto be associated with significant water flowsthat erode and entrain soil, shallow landslidesgenerally involve lower volumes of waterthan debris flows and are often triggered byelevated water pressures at the rock/soilinterface.Understanding the difference between

these event types is key to the provision ofsuitable tailored catch fences or barriersthat work to deal with the type of hazardeffectively and in a cost-efficient manner.The research and development of flexible

landslide barriers is now also well advancedwith a good understanding of the behaviourof both the landslide material and velocities(and hence energies), and of the barriers

themselves.One of these new-generation landslide

barriers was installed by BAM Ritchies earlierthis year, also at the A83 Rest-and-be-Thankful, on a 35° slope with challengingoverburden using deep piles and anchors. Thebar and rope anchors were drilled using arope-supported drill rig.

MMoonniittoorriinngg aannddiinnssttrruummeennttaattiioonnDevelopment of wireless and wiredcommunication systems, sometimes using themobile telephone network, nowadays allowsremote-sensing equipment to be installed atspecific slope locations as part of ongoingmonitoring solutions. Advanced laserscanning can be used to identify movementsor any changes that are of concern.New systems recently developed can

include overburden movement detection,measurement of water pressure at therock/soil interface and automated weatherstations. When a system records movement,an alert can be automatically sent to the clientallowing them to instigate additionalprotection measures or a halt to quarryactivities.Following a period of operation, the logged

data can be used to correlate shallowlandslide events with combinations of weatherevents and pore pressure behaviour, allowingan element of predictability and, thus, greateruser protection and maintenance efficiency.

CCoonncclluussiioonnThe Quarry Regulations 1999 require specificmeasures to be taken to ensure the stabilityand safety of excavations and tips. Carefulplanning and execution of working andresidual faces can achieve this. Where the faceis already in existence but cannot be stabilizedby scaling or re-profiling, there are other,modern, cost-effective solutions available.These solutions continue to evolve, thus assisting compliance with therequirements.For further information, contact David

Gibson on tel: (01275) 875338; fax: (01275) 870076; or email:david.gibson@BAMRitchies.co.uk QM

5,000kJ barrier certification test

Mining barrier protection

Haul road protection at KCGM Mine super pit, Australia